With Denny Hamlin and Kyle Busch both needing to pit for fuel late in the race Sunday at Dover, the scrutiny on Toyota’s engine program intensified as they failed to get the fuel mileage of some of their counterparts.

Toyota Racing Development took over Joe Gibbs Racing’s engine operation this season and provides engines for each of the Gibbs teams, as well as Michael Waltrip Racing.

TRD engines have powered Toyotas to eight victories this season—five for Hamlin, two for MWR’s Clint Bowyer and one for Busch.

But it also has had some engine problems, including three that sidelined Busch and contributed to him missing the Chase. Hamlin and Bowyer also had at least one engine failure each.

TRD president Lee White said Thursday that his organization has worked hard on the introduction of electronic fuel injection this year and stands by its record of eight failures on race weekends for the six cars (three at JGR, three at MWR) that it supports. He said that is better than the performance by Chevrolet’s Hendrick Motorsports but not as good as Dodge’s Penske Racing.

Hendrick has had five official engine failures during races among its four Sprint Cup teams, according to NASCAR statistics. Penske has had none, while Ford’s Roush Fenway Racing has had just one.

“We’ve been under extreme pressure from crew chiefs, teams and everybody—we’ve brought several performance upgrades to the racetrack,” said White, whose Toyota engines led 342 laps overall Sunday. “We have made tremendous progress in outright power and performance.”

Then there’s the fuel mileage issue. Teams can choose one of three levels of fuel mixture, and all six chose maximum performance over fuel efficiency at Dover, White said.

In his two victories this year, Bowyer’s MWR team chose fuel efficiency because its car was burning up its right rear tire with maximum horsepower.

“You never have enough horsepower,” White said. “Obviously any failure of any kind is too much because invariably it happens at the wrong time. And now, because of the nature of the racing in NASCAR, this whole fuel mileage thing has become a popular topic.

“It certainly is something that we’ve looked at. But to be honest with you, our focus with fuel injection has been number one, performance, number two, durability, and number three, fuel mileage.”

NASCAR implemented electronic fuel injection systems this season to replace carburetor-fueled engines it has run for years.

White said he is happy with the way TRD and JGR have merged operations. TRD had just been supplying engines to MWR and the now-defunct Red Bull Racing team before this season.

“Given that none of their organization had any experience with fuel injection, it was almost preordained that something like this had to happen,” he said. “… The Gibbs collaboration has improved the engines (we’re building).”